Since Dr. Raymond Damadian first suggested using nuclear magnetic resonance (NMR) techniques for detecting cancer, both in vitro and in vivo, in the early 1970's, the field of NMR scanning and imaging has rapidly developed such that today medical NMR imaging is an accepted and highly useful modality for the detection of various diseases and abnormalities. For instance, NMR imaging, or magnetic resonance imaging (MRI) as it is sometimes known, is known to be useful in the detection of neoplastic diseases (for example, cancer), vascular diseases (for example, aneurisms), degenerative diseases or disorders, traumatic disorders, congenital disorders, inflammatory disorders, and metabolic disorders as well as a number of other disorders or diseases. Indeed, NMR imaging, despite its relative infancy, is now believed to be one of the more powerful medical imaging modalities in terms of its ability to show abnormalities in human tissue and body fluids, even surpassing CT and X-ray imaging methods and techniques in many areas of the body such as the posterior fossa, the spinal cord and many soft tissues. The potential medical usefulness of NMR scanning and imaging is still being investigated and studied, and it is expected that the types of diseases or disorders for which NMR imaging is useful will increase, particularly as new imaging techniques and methods are developed.
Present day NMR scanning and imaging systems include highly sophisticated pieces of equipment. As is known, such equipment includes means for providing a primary static magnetic field of high strength and uniformity in a particular region thereof, as well as means for applying an orthogonally-directed oscillating magnetic field at particular frequencies (usually in the radio frequency or RF range) so as to produce NMR signals from selected nuclei of the patient or person positioned in the apparatus. Present day NMR imaging systems also include means for detecting such produced NMR signals, as well as suitable electronic controls and other equipment, such as computers, for processing of the derived signals to produce spatially encoded NMR image data, and for storing same for the later production of images based thereon. In this regard, with present day NMR imaging systems, the spatially encoded NMR data comprises NMR signal information and spatial information obtained during a scanning operation for a multitude of small volume areas in a selected region of the patient, generally known as small volume elements or voxels. For convenience of physicians and other medical personnel, the spatially encoded NMR data is then displayed in the form of a two-dimensional image or pictorial representation of the particular area of the patient for which the data is collected. In this regard, the images are produced by assigning a particular grey scale or brightness level to the NMR signal information for each of the small areas and then displaying such information in the form of a matrix. Thus, an NMR image is simply a visual translation of the derived NMR signal information obtained during the imaging scan or procedure into a grey scale.
As is well-known, the NMR signal information, generally the intensity or amplitude of the NMR signals measured during the scanning procedure, is a complex function of various tissue-related parameters. These tissue-related parameters generally are the spin-density of the particular nuclei being imaged (usually protons or hydrogen atoms in most medical NMR imaging applications), as well as the spin-lattice relaxation time (T.sub.1) and spin-spin relaxation time (T.sub.2), the latter two parameters both being exponential time constants which characterize the rate of return to equilibrium of the perturbed nuclei following the application of the perturbing RF or oscillating magnetic field. In this regard, the T.sub.1 and T.sub.2 information contained in the derived NMR signals is closely related to an indicative of the differences between normal healthy tissue and diseased or abnormal tissue, such that the resulting NMR images can be used by physicians and other medical personnel for the purposes such as might be caused by various types of diseases or disorders such as those noted hereinabove.
Present day NMR imaging techniques generally employ magnetic field gradients for encoding spatial information into the derived NMR signals so that data can be acquired in a relatively rapid manner for a large number of small volume regions or areas of the patient. Generally, with such systems, sets of orthogonal magnetic field gradient coils are provided in the apparatus for generating magnetic field components in the same direction as the static field, but whose strengths vary along the direction of the gradients. The application of such magnetic field gradients serve to change the magnetic field strength within the apparatus at various locations, thus changing the frequency of the applied oscillating magnetic field required for exciting the selected nuclei and/or the frequency of the derived NMR signals. Decoding of the spatially encoded information in the derived signals is then achieved through the use of two-dimensional and three-dimensional Fourier transformation techniques. Numerous such imaging techniques utilizing magnetic field gradients are known in the art, and are generally employed in connection with homogeneous or substantially homogeneous primary static magnetic fields.
With such present day NMR imaging systems, the scanning volume region of the apparatus, i.e. the region from which usable spatially encoded NMR image data is acquired, is of a limited size which is significantly less than the size of patients for which data is to be acquired. More particularly, with present day NMR image systems, NMR imaging data is generally obtained for a thin plane or slice of the patient or, in some instances, a series of thin planar regions stacked one on top of the other. It will thus be appreciated that NMR image data is collected from a three-dimensional array of a large number of small volume elements of particular finite dimensions. For instance, the three-dimensional array may comprise a three-dimensional array of rectangular geometry, in which the length and width is typically on the order of approximately 25".times.25", and the thickness or height is on the order of about 12". The two-dimensional NMR images are then produced from the image data for the series or sets of volume elements lying along a particular planar region within the three-dimensional array of rectangular geometry. The number of voxels or volume elements along any particular dimension of the rectangular prism-shaped volume can vary, but typically, may be on the order of either 128 or 256 elements along each edge. The resulting image will thus constitute a matrix having either 128 rows and 128 columns, or 256 rows and 256 columns. The size of the volume elements in plane in present day systems generally is 1 mm, but may vary for 0.5-2 mm. The slice thickness dimensions of the volume elements may vary independently, but typically is from 2-10 mm. Thus, very high resolution images are provided of the parts or regions of the patient for which the data is collected and displayed in the form of an image.
Since the scanning volume region of present day NMR imaging systems is of a limited size, it will be appreciated that in practice today, NMR imaging is used for examining particular localized regions of possible interest in a patient, as opposed to being used for examining the entire or substantially the entire body of the patient. Furthermore, in most instances, physicians only request and obtain NMR images (and only with respect to particular regions of potential interest) when there appears to be some basis for suspecting a particular type or types of disease or illness. In other words, based upon a physician's examination and interview of the patient, as well as the results of other tests which might be conducted on the patient, a physician may develop a number of potential or possible reasons for the patient's symptoms, and thus, order to request that NMR images be obtained in a suspected region or area of the patient. The NMR images are then used by the physician to determine whether particular abnormalities are present which are consistent with particular possible diagnoses, or to rule out certain causes or conditions symptomatic of a particular disease or illness. Thus, NMR images are generally only used with respect to patient's who are not healthy or who are suspected of being no healthy. Further, the NMR images generally are obtained for only particular regions or possible interest and are only used for purposes of determining whether there is any abnormality at the particular region of interest. They are not generally used in the manner of a medical screening or checkup procedure for purposes of screening both the healthy and unhealthy population to hopefully determine, at an early stage, whether the patient has a disease or is developing a condition indicative of disease or illness.
In some instances, NMR images have been used to screen patients who have a high potential risk for a particular disease or abnormality, such as cancer. For instance, NMR imaging may be used with respect to patients exposed to asbestos at some point in their lives, it being realized that such group of people have a high risk for developing cancer. In these instances, the patients may be scanned at intervals during their life to look for the occurrence of particular abnormalities which would be expected if cancer is present or developing. Again, however, the images are only obtained in those areas of the body where the particular type of cancer would be expected, and are only intended for determining whether a particular type of abnormality or condition is present.
Here it should be noted that since NMR imaging techniques do not employ ionizing radiation, such as is employed in X-ray or CT equipment, and have no known harmful effects to the patient, and further, since NMR imaging is known to be useful in detecting abnormalities caused by a vast array of different types of diseases or illnesses, NMR imaging has the potential to be a highly useful screening tool for physicians with respect to all areas of the body, if NMR scans and images could be obtained in a rapid and practical manner for virtually the entire body. To date, however, such total body scanning NMR imaging techniques for acquiring NMR image data in a rapid, practical and useful manner for substantially the entire body of a patient do not exist.
The reason for this is severalfold. First, present day NMR imaging techniques and apparatus are directed to obtaining high resolution images of particular localized regions of the body. Thus, the emphasis in present day developments has mainly been directed to reducing scanning times while, at the same time, obtaining the same or higher resolution images. The resolution capabilities of NMR images are inversely related to the size of the small volume elements for which individual NMR signal information is obtained, i.e., the smaller the volume element, the higher the resolution. Typically, the size of the volume elements for which NMR signal information is obtained is on the order of 3-30 mm.sup.3. Typically, NMR images comprise a matrix of 256.times.256 picture elements, or over 65,000 pixels or regions for which NMR image data must be acquired. With the desire to obtain high-resolution images in which the individual volume elements or regions are 3-30 mm.sup.3 in size, it will be appreciated that the resulting image will only constitute a display of approximately 10".times.10", although NMR images data may be acquired during the course of a scan for a somewhat larger area. With such NMR images, it will be appreciated that the number of images necessary to represent the entire body or substantially the entire body of the patient is very large and cannot practically be reviewed by a physician interested in screening a patient's body.
Another factor leading away from NMR imaging being used in connection with a general screening or checkup-type procedure is the fact that the NMR imaging apparatus presently used has only a limited scanning volume region from which NMR signal information can be obtained. Thus, in order to acquire NMR information with respect to the entire body utilizing present high-resolution imaging techniques, a large number of separate scanning operations would be required for each different region of the body. This, in turn, requires movement of the patient, and also, performing new setup operations for each separate scan. These factors all increase the time for acquiring the overall NMR image data for all areas of the body.
Furthermore, even if the data is acquired for the entire body, the images which are produced are images of only selected regions of the body, namely, regions lying in the different portions of the body for which each separate scanning operation is performed. In other words, while techniques have been developed for acquiring large amounts of NMR imaging data, and in relatively fast scanning times, with present day techniques this is only useful for producing a large number of separate images for each separate region or portion of the body which is scanned. As can be appreciated, with the present day desire to only obtain high-resolution images, the number of regions for which spatially localized NMR imaging data has to be acquired significantly increases such that the number of NMR images required to be reviewed is so great as not to be practical for use by physicians for performing a general screening or checkup-type technique.
Thus, while NMR imaging techniques might be useful in connection with a general medical screening application, such techniques are not presently practical for use in connection with a total or substantially total body scan of a patient. In essence, with present day NMR imaging techniques, the number of NMR images required are so great as not to be practical for use by physicians in performing a general screening or checkup-type procedure, the individual images produced only being representative of selected regions of the patient for which NMR image data is acquired in separate scanning operations and the number of images required for representing the entire or substantially the entire body being very great in number.